Wireless communication systems transfer data from a transmitter (TX) of one station to a receiver (RX) of another station. The TX and RX can be ground-based, airborne or spaceborne. Furthermore, multiple stations (TX or RX) can be ground-based and in communication with one or more air or space platforms (RX or TX). These ground/airborne/spaceborne telecommunication systems have to support uplink and downlink of large and ever-increasing volumes of data (e.g., Internet data).
To address higher capacity and higher performance needs, the fiberoptics industry developed coherent fiber optic transceiver technologies using digital signal processors (DSPs) for the next generation of high-rate communications. DSP-based coherent transceivers offer the advantage of an intrinsic 4 to 5 dB signal-to-noise ratio (SNR) improvement over intensity modulated direct detection (IMDD) systems using optical preamplification, along with implementation factors that, in practice, increase performance by another 5 to 6 dB. These transceivers also offer higher spectral efficiency and lower power consumption. Moreover, the integrated photonics technology employed in coherent transceivers provides a competitive $/bit/s cost-benefit value, e.g., as can be measured in dollars per bit per second.
Errors in recovering wirelessly transmitted data may be due to many causes, including but not exclusive to noise sources in the receiver and distortion of the signal in the transmission medium. The plurality of causes, referred to as channel impairments, are typically mitigated by a combination of techniques including channel equalization and forward error correction (FEC) coding at the physical layer. Channel equalization is used to reduce the amount of inter-symbol interference induced by band-limiting in the receiver or channel to produce better estimates of the received symbols. Physical layer forward error correction (FEC) coding introduces a structured redundancy on the transmitted symbol sequence that can be exploited at the receiver to correct errors in recovering the transmitted data due to channel impairments.
In free-space optical communications systems that propagate light through air, a significant source of channel impairment is turbulence. Turbulence causes frequency nonselective fades in optical power. The fade process has a correlation time which is typically much longer than the duration of a symbol.
To reliably communicate in the presence of a frequency nonselective slowly fading channel, temporal diversity techniques can be used. In this technique, multiple observations of the transmitted data are made across distinct channel coherence times can be used. This can be accomplished with groups of physical layer FEC encoded symbols (referred to herein as “codewords”) that span multiple channel coherence times, codewords that are interleaved to span multiple channel coherence times, or automatic-repeat request (ARQ) schemes that, upon receiving a block of data in error, request a re-transmission of that data. The encoding and decoding complexity of a physical layer FEC code increases with the length of the codeword, and for high data rate systems it can be prohibitively complex to have a codeword span multiple channel coherence times. Channel interleaving techniques interleaves the symbols from multiple codewords to extend the support of each across the length of the interleaver. This method (in the limit of long interleaver length) achieves the channel capacity; however, depending on the type of interleaver and physical layer FEC code, this method can incur a fixed latency penalty that is undesirable or exceeds the system requirements. An ARQ scheme adapts with the channel conditions and therefore does not incur a fixed latency penalty; however, the additional round-trip latency from the re-transmission requests and the need for an additional feedback channel make these ARQ schemes impractical or undesirable in many cases.
Another conventional approach to mitigate fading relies on spatial diversity. Turbulence has a transverse correlation length r0. If two optical source beams are separated by a distance D, then their fades will become independent when D>>r0. Turbulence also causes optical phase to be randomized, with similar correlation times and distances. A conventional technology that utilizes spatial diversity to mitigate turbulence is called multi-beaming. The multi-beaming technique works by sending the same symbol along different paths separated by D, where D>>r0 such that different paths experience statistically independent fades and phase offsets. When the various signals are highly non-coherent to each other, the intensity average over a symbol period is equal to the sum of individual path signal intensities. In this case, the total received signal intensity is the sum of several independent fading processes and will thus have reduced fade frequency and fade depth. This approach is suitable when information is encoded only in intensity, but is not applicable when the optical phase carries information. Accordingly, there remains a need for optical communication systems that can reliably transfer data through the atmosphere with high data throughput and low latency of the data transmission.